The present invention relates generally to the field of photographic film image scanners and more specifically to calibrating image scanning apparatus in such scanners.
Photographic film scanners for converting optical images on a film strip into files of digital data on a pixel-by-pixel basis commonly employ a solid state image sensor having an array of pixel sized photosites. Before scanning individual image frames on the film strip, it is necessary to perform a calibration procedure in which signal outputs from individual photosites are adjusted to eliminate effects in the scan system that adversely affect the accuracy of the signal outputs in representing pixel density values of the scanned image. A typical solid state photosensor of this type is a charge coupled device (CCD) imager containing an array of light detecting photosites which accumulate charge depending on the light energy projected onto them. After some charge accumulation time, the charges in the photosites are transferred to a charge shifting structure so that the charges may be shifted out of the CCD and measured by a signal processing circuit in order to form an image signal representative of the image projected onto the CCD. Because of such things as manufacturing variability in the CCD, dust or contaminants in the optical path which projects an image onto the CCD, light source non-uniformity, or other source of variations, the system response for individual photosites may not be the same from pixel-to-pixel of the image. Compensation for this pixel to pixel variation may be provided in the charge measuring process. It is known that this compensation can be provided by multiplying the output value for each photosite by compensating gain and offset values. Offset compensation is used to adjust for different photosite dark current (no illuminating light) characteristics. Gain compensation is used to adjust for different photosite sensitivities as well as to eliminate perturbations in the sensor output caused by anomalies in the optical scanning path, such as dust particles on the film strip and the like.
Typically, the system response for a given imaging pixel does not change in the short-term. Hence, the gain and offset values required to adjust the system response for given imaging pixels back to some ideal response can be determined by a calibration process and then applied whenever the signal for the imaging pixel he shifted out of the CCD. A typical calibration process obtains samples of the system response for each imaging pixel at some nominal signal input level with some nominal gain an offset values and then calculates the required gain and offset values for each of the imaging pixels.
In a photographic film scanner, it is common practice to perform a calibration of the image scanner at the beginning of scanning of images on a strip of photographic film. This calibration setup, referred to herein as gain and offset calibration profiles, is then used in the scanning of successive images on the film strip. However, when a long film strip with as many as 40 images on the film strip, the time to complete scanning of all of the images on the film strip can take several hours. During this time, significant changes in operating conditions of the scanner can cause the initial scan calibration to in accurately reflect necessary compensation to achieve proper output signal. The system can drift due to thermal variations, such as gradual heating up of the scanner components after initial power-up and due to changes in ambient conditions, for example, sunshine on the scanner. There is therefore a need for a calibration arrangement for a photographic film scanner which can adjust the calibration settings for scanning operations that will compensate for changes in system conditions throughout the scanning operation.
In accordance with the invention, there is provided a method of scanning a film strip and establishing image frame-specific gain calibration settings for a film image scanner including a solid state image sensor including an array of photosites wherein the method comprises obtaining and setting, in advance of commencing image scanning operation, an initial gain calibration profile for gain adjustment, on a photosite-by-photosite basis, of output signals from said array of photosites and obtaining, at substantially the same time as said initial gain calibration profile is set, a reference film Dmin value from at least one predetermined photosite aligned with a region of film pixels extending longitudinally of the film strip outside of image frame areas of the film. The method includes subsequently advancing the film strip to one or more image frames to be scanned; obtaining, for each frame to be scanned, a frame-specific Dmin value from said at least one photosite obtained from said region adjacent to each said image frame to be scanned and then setting a revised gain calibration profile for each image frame scan based on adjustment of said initial gain calibration profile in proportion to differences between said reference Dmin value and said frame-specific Dmin value corresponding to the image frame to be scanned.
These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.